Sea urchin sperm can be obtained in large quantities,
because sea urchins, like many marine invertebrates,
engage in broadcast spawning, i.e., they release
large quantities of gametes into the sea water. Undiluted
semen contains a large quantity of sperm - as many as
1010 - 1011 cells/ml!

Sperm swim by means of a prominent flagellum,
composed of a core of microtubules, whose sliding
is powered by flagellar dynein. This array of
microtubules and associated motor and linker proteins is
known as an axoneme. The midpiece of the sperm
contains a prominent array of mitochondria, which
are required to produce huge amounts of ATP, whose
hydrolysis powers the conformational changes in flagellar
dynein that mediate microtubule sliding.
Image courtesy of Thomas Gensch, Forschungszentrum
Jülich.

Most flagella, like cilia, have a characteristic
“9+2” structure, i.e., two central singlet
microtubules are encircled by nine outer doublet
microtubules. The outer and inner dynein arms slide along
each outer doublet microtubule. The following diagram
shows how the inner and outer doublet microtubules of the
axoneme are connected to dynein.

Internal structure of the
axoneme. Adapted from an image courtesy of Takashi Ishikawa, Swiss Federal
Institute of Technology, Zurich.

Sea urchin sperm move their tails using a helical motion,
as shown here in this animation.

Finally, as the movie below shows, flagellar bending and
the sliding of microtubules can be examined by analyzing
the movements of small beads placed along the surface of
a flagellum treated with detergent. The beads move apart
as the wave of flagellar bending passes by, indicating
that flagellar bending is mediated by microtubule
sliding.

Description: This
sea urchin spermatozoon was treated with detergent to
remove the cell membrane, and then transferred to a
solution containing a relatively low concentration of
MgATP, to reactivate the bending of its flagellum. Before
reactivation, the spermatozoa were exposed to a
suspension of 40 nm gold beads. Some of these beads
attach to the outer doublet microtubules that comprise
the flagellar axoneme. In this case, two beads were
attached to doublet microtubules on opposite sides of the
flagellum. As the flagellum bends, the sliding between
these doublet microtubules is demonstrated by the
movement of the two beads. Movie courtesy of Charles Brokaw, California
Institute of Technology.